First‐order major element variation in basalt glasses from the Mid‐Atlantic Ridge: 29°N to 73°N

Abstract

Basaltic glasses from 29°N to 73°N on the Mid‐Atlantic Ridge define two geographic and compositional groups that are characterized by different major element concentrations, phase assemblages and partition coefficients. Group A glasses occur on the ridge segment north of Gibbs Fracture Zone, from 54°N, along the Reykjanes Ridge, the western volcanic zone of Iceland and Kolbeinsey Ridge, up to 70°N. Similar glasses occur also on the ridge from 29°N to 34°N. Group A is characterized by pronounced alumina depletion and iron‐enrichment trends, lower silica and alkalis and higher iron and high CaO/Al2O3. Clinopyroxene is only present in the more evolved group A glasses (Mg‐value < 58). Olivine‐glass partition coefficients for Mg and Fe are systematically lower in group A than group B (Schilling and Sigurdsson, 1979), reflecting effects of melt composition on structure of the liquids. Group B glasses define the ridge segment north of 35°N over the Azores platform and up to Gibbs F. Z., at 53°N. They are also present on the Mohns Ridge north of the Jan Mayen F. Z. at 71°N and at least as far north as 73°. Group B glasses are characterized by higher silica and alkalis, low iron and lack of alumina depletion and iron‐enrichment trends. They contain calcic clinopyroxene (Cpx) throughout the compositional range, whereas the sub‐calcic augite is absent. The major‐element variation within each group can be modeled quantitatively by fractional crystallization involving the phenocryst phases. In group A, the model involves only olivine (Ol) and plagioclase (Pl) extraction in the early stages, but a total of 60% crystallization of the most primitive magma is required, yielding a final solid extract of Ol‐Pl‐Cpx in the proportions 1:3.3:1.5. Group B fractionation models require clinopyroxene extraction throughout the compositional range, and 45% crystallization of the primitive magma is required to account for the most evolved glasses giving a total solid extract of Ol‐Pl‐Cpx in proportions 1:5:3.9. The range of glass compositions in the two groups forms two distinct but parallel cotectic trends within the basalt tetrahedron, which are believed to define the quaternary univariant line Ol‐Pl‐Cpx‐Liq along which the magmas evolved during fractionation at low to intermediate pressure. It is proposed that the shift in phase boundaries and univariant lines reflects differences in melt and source compositions of the two groups. The distribution of the two groups is thus taken to indicate two major petrographic provinces of the Mid‐Atlantic Ridge.